The present invention relates generally to Code Division Multiple Access systems and, more particularly, to power control methods useful in Code Division Multiple Access systems.
Numerous access schemes exist to allow multiple users to share a communication medium. One such access scheme is known as Code Division Multiple Access (CDMA). CDMA is a form of multiple access employed by spread-spectrum communication systems. In CDMA systems, a wideband spreading signal is used to convert a narrowband data sequence to a wideband signal. The spreading signal typically comprises a pseudo noise (PN) sequence that has a chip rate several orders of magnitude higher than the data rate of the data sequence. The resulting wideband signal occupies a bandwidth in excess of the minimum bandwidth required to transmit the data sequence.
One spread spectrum technique employed in CDMA systems is known as direct sequence spread spectrum. In direct sequence spread spectrum systems, the data sequence modulates a PN sequence, which serves as the spreading signal, to generate a wideband signal. Modulation may be achieved, for example, by applying the data sequence and PN sequence to a product modulator or multiplier. Multiplication of two signals produces a signal whose spectrum equals the convolution of the spectra of the component signals. If the data sequence is narrowband and the PN sequence is wideband, the product of the data sequence and PN sequence is a wideband signal whose spectrum is nearly equal to the spectrum of the PN signal. Thus, the PN sequence functions as a spreading signal.
In CDMA systems, multiple users may use the same carrier frequency and may transmit simultaneously. Each user has its own PN sequence which is approximately orthogonal to the PN sequences of other users. Transmissions to or from individual users are imprinted with that user's PN sequence. The receiver selects the desired signal, which combines in the communication channel with unwanted signals, by performing a correlation operation. That is, the receiver correlates the received signal with the PN sequence of the desired signal. All other signals are spread by the PN sequence and appear as noise to the receiver.
Power control is used on the reverse link in CDMA systems to control the power of signals received at the base station. The purpose of power control is to assure that each mobile terminal served by a particular base station provides approximately the same signal level to the base station receiver. In CDMA systems, the system capacity is maximized if each mobile transmitter power level is controlled so that its signal arrives at the base station receiver with the minimum required signal-to-interference ratio (SIR).
The current standard for CDMA systems in the United States is contained in a specification published by the Telecommunications Industry Association and Electronics Industry Association (TIA/EIA) as IS-95. New standards for wideband CDMA are currently being developed in North America, Europe, and Japan, which offer significant performance improvements compared to the current CDMA standard. One of the new features that contributes to the improved performance of wideband CDMA is fast forward link power control (FFLPC). FFLPC combats Rayleigh fading in the forward link propagation channel at slow user mobility. Power control for fast user mobility is not easily achievable within the current CDMA standards.
In the new CDMA standards, forward link power control is implemented by the base station with the aid of the mobile terminal. The base station commands the mobile terminal to maintain a predetermined signal quality standard, such as a specified frame error rate (FER). The mobile terminal maps the target FER into a target signal-to-interference ratio (SIR) that is required to achieve the target FER. The mobile station estimates the SIR continuously and sends power control bits to the base station multiplexed into the reverse pilot channel. If the measured SIR is below the target SIR, the mobile station sends a power control bit (PCB) with a value of +1 to request an increase in its transmit power. If the measured SIR is above the target SIR, the mobile terminal sends a PCB with a value of −1 to request a decrease in its transmit power. In response, the base station may increase or decrease its transmit power on the forward traffic channel assigned to the mobile terminal by a value ΔP. The base station operates at the new power level for a period of time T until it receives a new power control bit from the mobile terminal. Normally, the time T is the duration of one power control group (PCG). Each forward traffic channel frame, which has a duration of 20 milliseconds, consists of sixteen PCGs, each with a duration of 1.25 milliseconds. A PCB is transmitted during each PCG so that a total of sixteen PCBs are transmitted in a frame.
Another new feature in the emerging American standard for wideband CDMA, which does not exist in the emerging European standard, is the introduction of quasi-orthogonal codes (QOCs). Under the current American standard for CDMA, when a mobile terminal makes a connection, it is assigned one code chosen from a set of 64 Walsh codes to use during the call. Walsh codes are orthogonal when they are synchronous. Hence, if there is no multipath in the propagation channel, the forward traffic channels from the base station to all users in the cell are orthogonal, and users in the same cell do not cause interference with each other.
The current CDMA standard is used primarily for voice communications. The new CDMA standard is geared more toward high data rate and multimedia applications. Under the new standard, a single user can simultaneously receive many data streams corresponding to many services, such as voice, video, internet, fax, etc. To provide all these data streams, the new American CDMA standard allows the cellular system to assign up to three codes, corresponding to three forward traffic channels, to the same user. With the increasing demand for cellular services, the number of users is expected to rise. Thus, the new CDMA standard needs more codes than is provided by the standard set of 64 Walsh codes to satisfy the increased number of users and the expected increase in the number of simultaneous codes per user. To meet this demand, the new CDMA standard has added a set of quasi-orthogonal codes. These code sets have the characteristic that codes in the same set are orthogonal. However, two codes from different sets are not orthogonal and, when used together in the same cell, cause interference with each other.